1. Field of the Invention
This invention relates to mercury-cadmium telluride (HgCdTe) MIS arrays, and, more specifically, to a method of protecting against electrostatic discharges across the dielectric during array fabrication which cause damage and/or imprefections in the array elements.
2. Description of the Prior Art
MIS array elements are susceptible to charge buildup and ultimate catastorphic breakdown in the insulator due to the voltage that can be built up because the elements are capacitors and include insulators which are not perfect and have a breakdown voltage. Charges on these capacitors can be picked up anywhere during processing or handling of the elements themselves. Prior art MIS elements of the type involved herein normally include a HgCdTe substrate which is one plate of the capacitor and which has an oxide passivating layer thereon, further insulating layer of zinc sulfide over the oxide layer and a gate electrode over the insulating layer. This is the basic unit of all infrared focal plane imagers. The gate forms the other plate of the capacitor and any charge built up thereon which is too large will break down the dielectric and cause a short circuit. The problems encountered by this action are not only catastrophic failure noted above but also the problem that, though a short circuit does not exist, the insulating layer may have been damaged to the extent tha the flat band voltage varies due to a voltage across the insulator during manufacture which is not enough to cause breakdown, but is enough to move the fixed charge in the oxide layer and in the active region therefrom. Furthermore, due to the problem of breakdown as described hereinabove, the prior art MIS arrays are normally probed to locate faulty elements. This also presents a problem in that the function of probing itself causes breakdown of elements to some extent, thereby further reducing the ultimate yield. The above noted problem exists only in HgCdTe and similar type compounds wherein zener diodes (avalanche conduction at a voltage above MIS operating voltage but below MIS breakdown voltage) cannot be formed as in the case of silicon and the like. Accordingly, the solution to the problem described hereinabove can not be provided by the addition of a diode between the bonding pad and the HgCdTe substrate as is done in silicon and other materials wherein diodes can be formed in the substrate itself.